Abstract : Nanocomposite materials attract search due to their improvements on barrier properties by incorporating low level of nanofiller of 5%w. Nowadays, organically modified montmorillonites MMT-O are the most used fillers due to their high aspect ratio which permits stronger clay-polymer interactions. If nanoreinforced materials are highly performing, the ways in which clay presence affects polyolefin durability have not being subject of a rigorous study, thus they are not yet clear. Our goal was to examine unstabilized clay polypropylene and unstabilized clay polyethylene nanocomposites to get a better comprehension of the clay effects on their thermooxidation process under low temperatures. The effects induced by a dual physic -chemical nature of the clay were explored. The problem was tackled from both experimental and theoretical point of views for degradation process not controlled and controlled by oxygen diffusion homogenous and heterogeneous respectively. It seems that MMT-O speeds up oxidation. This phenomenon was modeled by adding a catalytic reaction between metallic particles initially present in the MMT-O and hydroperoxyde groups main responsible of oxidation. Regarding the oxygen permeability two situations were confronted: for the clay polypropylene system a decrease of 45% of oxygen permeability was measured. On the other hand, no variation was found for the polyethylene case. This effect was attributed to the fact that polyethylene nanocomposite reached a blend morphology less developed than those of the polypropylene nanocomposite. Kinetics and oxidation products profiles across the sample thickness were simulated for both systems by coupling oxidation reactions with oxygen diffusion equations. For the polyethylene case, the effects induced by oxidation on molar mass and crystalline morphology were also simulated. Finally, based on structure - property relation, simulations of mechanic modulus profiles were performed for the heterogeneous degradation case oxidation diffusion controlled. These results were confirmed by experimental measurements of modulus across the thickness of thick clay polyethylene nanocomposite samples.